cabriole

A cabriolet is a two-wheeled carriage, rather in the manner of a chariot,
drawn by a single horse, a small vehicle for a trip of 500 miles. Since
Victor mentions "horses" in the plural, perhaps Mary Shelley intended to
allow him a larger vehicle for his journey than the type she names. In
circumstances like this, the traveller would ride post, which is to say,
would change horses at stages along the road.